75: Stingers
Grand Theft Auto V Guide

Way to unlock: You need to complete main mission 74 (The Big Score) and select the subtle variant towards the end of that mission. After you receive a message from Lester, there will appear the Hs icon on the world map, which symbolizes the police van.

Playable characters: Michael

Additional information: The only premise of the preparatory mission is that you need to acquire the stingers which are inside the van. These are going to be during the heist to stop the armored vans.

The van that you are looking for is on the parking lot adjacent to the police station in the Rancho district, In the Southern part of the city. I strongly recommend that you try to sneak into the police parking lot, thanks to which you will not have to worry about the police pursuit and about whether you are going to make it with the stolen car without serious damage, or not. Try to make it to the back of the building, but do not pick the alley with the two policemen in it, and take the one neighboring it. Enter the sneak mode and, carefully, approach the fence shown in the above screenshot.

The van

Hop over the fence and go straight ahead, still while sneaking, and sneak past one of the policemen. Get into the police van (Police Transporter) and drive it out of the parking lot carefully, while trying not to get spotted. The only thing left for you to do is reach carefully the hideout and park it in the marked area.

Rewards for the completion of the mission: none

Requirements for the completion of the mission in 100%:

Escapee - you need to steal the van within two minutes. If you are quick enough, even half as much will do.